Comorbid substance use disorder (SUD) and depression disproportionately affect ethnic minorities, and is associated with elevated rates of post treatment relapse to substance use, HIV risk behavior, and associated poor mental and physical health outcomes. Although efficacious, the often complex, specialized nature of CBT poses problems in its integration into substance use treatment programs. Moreover, budget cuts for mental health and substance use treatment reduce availability of publically funded treatment programs and staff to patient ratios. To address this limitation, a behavioral activation (BA) treatment, the Life Enhancement Treatment for Substance Use (LETS ACT), was developed to treat depressive symptoms among a predominantly African American sample of low income illicit drug users currently receiving residential substance use treatment. Empirical evidence indicates that LETS ACT is associated with significantly better outcomes then a control condition for treatment retention, post treatment abstinence, HIV sexual risk behavior, depressive symptoms, and environmental reward. Although these strong outcomes suggest that LETS ACT may be ready for a Stage III dissemination trial, the effects of LETS ACT on abstinence were not significant beyond a 3- month post treatment follow-up, and there was a significant indirect effect of LETS ACT homework compliance on post treatment substance use and HIV sexual risk behavior via environmental reward. These findings point to the need to identify cost-effective delivery-vehicles to increase treatment engagement outside of clinician sessions. Finally, identifying neuroscience based biomarkers (neuromarkers) underlying key theoretical aspects of BA (i.e., reward sensitivity), and their relation to heterogeneity in BA treatment response among substance users with depression are critical for the identification of accurately targeted interventions. The current proposal will expand upon findings in R01 DA026424 by addressing the critical needs of depressed low income substance users to (1) determine if a smartphone enhanced treatment for LETS ACT (LETS ACT-SE) is associated with increased long term treatment engagement, and in turn, improved long term treatment outcomes, and (2) use functional magnetic resonance imaging (fMRI) to identify the effects of LETS ACT-SE on the neuromarkers of reward sensitivity. The application cuts across multiple NIDA initiatives aiming to (1) improve behavioral interventions using innovative technologies to boost effects and increase implementability (PA-13-077), (2) develop effective interventions to prevent new HIV infection (PA-12- 281), (2) identify the underlying mechanisms that impact HIV risk (PA-14-061), (3) identify neural circuits underlying substance use comorbidity with psychiatric disorders (PA-14-026), and (4) identify neurobiological mechanisms underlying drug abuse to improve treatment (PA-13-338).